The present invention relates to a scanning electron microscope for obtaining a scan image by scanning an electron spot on a sample, particularly to a scanning electron microscope capable of obtaining a scan image with a high spatial resolution.
A scanning electron microscope has been conventionally used for observation and length measurement of submicron-order (1 micron or less), such as contact holes and line patterns in a semiconductor device sample. The scanning electron microscope obtains a scan image (SEM image) by scanning an electron beam emitted from an electron source on a sample to detect a secondarily-obtained signal (secondary electrons and reflected electrons) and using the secondary signal for the brightness modulation input for a cathode ray tube scanned synchronously with the electron beam scanning.
A semiconductor device sample is generally made by forming an electrical insulation such as SiO2 or SiN on a conductive portion of A1 or Si. When an electron beam is applied to the semiconductor device sample, the surface of the electrical insulation is negatively electrified (hereafter expressed simply as "charge-up") and secondary electrons generated at the bottom of a contact hole are prevented from rising by negatively-electrified electric charges on the sample surface. Therefore, the secondary electrons cannot be detected by a detector. As a result, abnormal contrast or extreme distortion may occur in the SEM image.
The above image defects due to charge-up seriously affects the observation of contact holes and length measurement of lines-and-spaces. Therefore, this makes it difficult to not only evaluate semiconductor manufacturing processes but assure the quality of semiconductor devices. For this reason, a so-called low acceleration SEM has been conventionally used in which the energy of the primary electron beam applied to a sample is 10 keV or lower.
Because scanning electron microscopes have been used for semiconductor manufacturing processes and finished-product inspection processes (e.g. inspection of electric operations by an electron beam) in recent years, a high resolution of 10 nm or less at a low acceleration voltage of 1,000 V or lower has been required in order that an insulation can be observed without electrification.
However, if the acceleration voltage lowers, observation at a high magnification gets difficult because the resolution is extremely degraded due to chromatic aberration caused by variation of electron beam energy. If electron current decreases, the ratio of secondary signal to noise (S/N) extremely decreases, the contrast of an SEM image is impaired, and observation at high magnification and resolution becomes difficult. Especially for a semiconductor device made by an ultrafine processing technique, signals generated from recessed portions of contact holes and line patterns become weak. Therefore, this makes minute observation and length measurement very difficult.
To solve the problem, one approach (see description in "Some Approaches to Low-Voltage Scanning Electron Microscopy" by Mullerova, et al., Ultramicroscopy 41, 1992, page 399-410) provides a scanning electrical microscope capable of increasing the energy of the electron beam applied to the sample, caused by generating a decelerating electrical field, by applying a negative voltage to the sample, so as to keep the acceleration voltage comparatively high.
However, in the above art, because negative voltage is applied to the sample, the secondary electrons emitted from the sample are prevented from rising. Therefore, the problem of so called charge-up occurs.
To solve the problem, for example, Japanese Patent Laid-Open No. 22339/1991 discloses a scanning electron microscope capable of decreasing chromatic aberration by applying positive voltage to a channel cylinder of an electrooptical system to give high energy to an electron beam passing through a lens and by generating a deceleration electric field between the channel cylinder and a sample to decelerate the electron beam to be applied to the sample.
In the art, because the deceleration electric field is provided between the channel cylinder and the sample liner tube, the sample is capable of being prevented from applying the negative voltage.
However, the above prior art cannot be used because the publication does not describe any concrete secondary signal detection method. That is, the above art requires the application of a very high acceleration voltage (approx. 9,000 V) to secure a sufficient electron current because it uses a thermal-emission-type electron gun as an electron source. Therefore, the primary electron beam has a high energy (e.g. 10 keV). Thus, the ratio of energies of the primary electron beam before and after deceleration by a deceleration electric field is decreased and resultingly, the ratio of the energy of the primary electron beam to that of the secondary signal is decreased. Thereby, it is difficult to selectively acquire secondary signals.
An object of the present invention is to provide a scanning electron microscope capable of detecting a secondary signal emitted from a sample to obtain a scan image with a high spatial resolution.
This and other objects of the present invention are achieved by providing a scanning electron microscope for scanning an electron beam on a sample, and obtaining an image on the basis of a secondary signal produced from said sample, comprising an electron source for emitting the electron beam and a channel cylinder to which is applied positive voltage. The channel cylinder has a portion proximate to the sample, the channel cylinder portion having an electron source side and a sample side. The channel cylinder is between the electron source and the sample and a detector for detecting the secondary signal, and the detector is located on the electron source side of the channel cylinder, wherein the sample is grounded.
With the above embodiment of the present invention, chromatic aberration is decreased because the primary electron beam passing through a lens is given a high energy due to the acceleration voltage generated by the channel cylinder, and the primary electronbeam applied to the sample is decelerated by the deceleration electric field that is generated. Therefore, charge-up of the sample surface is prevented.
The secondary signal produced from the sample is accelerated in the upward direction between the sample and the channel cylinder, because the direction of the beam emitted from the electron source is opposite from the direction of the secondary signal produced from the sample. The secondary signal is forced to rise upward by the deceleration field, so as to move upward to the detector which is located on the electron side of the channel cylinder. Therefore, the detector can easily detect the secondary signal which is moved close to the detector due to the deceleration field.
To achieve the above objects, certain embodiments of the present invention comprise a channel cylinder which is provided to cover an electron channel extending from a field-emission-type electron gun to an objective and which forms an acceleration electric field in the channel cylinder, a deceleration electric field forming means for forming a deceleration electric field for a primary electron beam between the objective and a sample, and a secondary signal detecting means for detecting secondary signals attracted into the channel cylinder on the electron source side of the objective.
Because the field-emission-type electron gun is used as the electron source, the energy of the primary electron beam is decreased and the ratio of the energy of the primary electron beam before the deceleration by the deceleration electric field to that after the deceleration is increased. Therefore, the ratio of the energy of the primary electron beam to that of the secondary signal is increased and secondary signals can easily be acquired selectively.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.